being hurt doesn't excuse using your own children to get back at a GM because she's been seeing her other GC
It may not excuse it in your eyes but it does explain it. It is a given, however much the OP might wish it was not. DIL is deeply hurt by her action - that is a fact and cannot be altered without OP working on a new relationship with her.
As I have said before, standing on principle and dictating what people should do and should feel is the way to disaster.
Accepting the situation as it is (however bad OP might feel about it, and whatever her judgement might be of what should happen) is the only way forward.
Judging this DIL will get her nowhere at all; trying to understand her hurt is the only way forward.
In my experience standing on principle is death to good relationships.
Name, Place, Animal, Object 10
Have any of you got all electric cars? Pros and cons please.